Hotspots and development frontiers of postoperative complications of AD: Bibliometric analysis – a review

The research on the postoperative complications of aortic dissection (AD) has received great attention from scholars all over the world, and the number of research articles in this field has consistently increased year after year. However, no bibliometric reports have been published yet to analyze the scientific output and the current situation in this field. The Bibliometrix R-package, VOSviewer, and CiteSpace software were used to conduct a bibliometric analysis of the hotspots and development frontiers of AD. A total of 1242 articles were retrieved. The USA, China, and Japan had the highest number of publications. The five keywords with the highest frequency were “analysis,” “incidence,” “acute type,” “graft,” and “risk factor.” The results also indicated that the research in related fields had shifted from surgical treatment and utilizing experience to the evidence-based exploration of risk factors and the construction of prediction models to help better manage postoperative complications of AD. This is the first bibliometric analysis of global publications on the postoperative complications of AD. The current research hotspots focus on three areas: common postoperative complications of AD, exploration of the related risk factors, and management of complications. Future research could focus on identifying risk factors through meta-analysis and using a multicenter database for AD as well as building relevant models to predict the development of complications to better facilitate the clinical management of AD patients.


Introduction
Aortic dissection (AD) have become one of the main diseases threatening the life safety of patients because of its dangerous onset, rapid progression, diverse initial symptoms, and high misdiagnosis rate. In addition, the mortality rate for AD is high, with an overall in-hospital mortality of 27.4%; the in-hospital mortality rate for type A dissections is 26% to 58%, while that for type B dissection is 11% to 31%. [1] Patients who present early, untreated acute type A aortic dissection (AAAD) die at a rate of 1% to 2% per hour on the first day, and almost half of them die at the end of the first week. [2] Urgent open surgical repair is the treatment of choice for AAAD, whereas type B dissection is usually treated with medical therapy, thoracic endovascular aortic repair (TEVAR), or surgery depending on the severity of the disease. [3,4] In recent years, new studies have indicated that the overall mortality for AD decreased in Australia, the United States, and many European countries between 2000 and 2017. [5] However, despite improved diagnostic and therapeutic techniques, postoperative complications remain an independent risk factor for in-hospital mortality. [6] As type A dissections involve the ascending aorta, aortic arch, and descending aorta, surgery is often complex and has a high surgical risk. [7] It is prone to complications such as hypoxemia, [8] acute respiratory distress syndrome, [9] delirium, [10] renal insufficiency, [11] cardiac insufficiency, [12] impaired neurological function, [13] and paraplegia. [14] Type B dissections are often treated using TEVAR. Although less invasive and having a shorter time frame, postoperative complications such as endoleaks, [15] secondary type A dissection, [16] stroke, [17] and paraplegia [18] can still occur. Further, AD results in different complications for different patient groups, such as the elderly, children, and pregnant women. [19][20][21][22][23] Recently, the research on the postoperative complications of AD has received considerable attention from scholars worldwide, and the number of research articles in this field has increased annually. However, no bibliometric reports have been published yet to analyze the scientific output and the current situation in this field. Therefore, it is important to adopt visual methods to illustrate the global status, research hotspots, and development frontiers of the postoperative complications of AD. Using the Bibliometrix R-package, VOSviewer, and CiteSpace software, we conducted a bibliometric analysis of relevant studies on the postoperative complications of AD and provide a new perspective for future research to improve the clinical management, prognosis, and rehabilitation of patients who present with AD.

Data source and search strategy
A bibliometric literature search was performed online using the Web of Science Core Collection (WoSCC) on May 27, 2022, with the time span for publication set to 1991 to 2022. Three Medical Subject Headings terms were used for the search. Term A was "aortic dissection," Term B was "complication," and Term C was "postoperative." The search criteria for the included publications were the following: TS = (aortic dissection) AND TS = (complication) AND TS = (postoperative). Among the types of documents included were original research articles and reviews (including meta-analysis) written in English. The exclusion criteria were as follows: repeated published literature; and non-English literature. A total of 1242 articles were included in the study, and the literature was exported and saved. All records and references were formatted and stored as plain text files in.txt format.

Data analysis
Visual analysis was performed using the Bibliometrix R-package, VOSviewer, and CiteSpace software. The R tool Bibliometrix (Version 3.2.1) of R-Studio (Version 4.1.0) was used to carry out a comprehensive bibliometric analysis, combined with the web-based tool Biblioshiny, to export and manage the data from WoSCC. [24] Its basic functions include an analysis of the main information, average number of citations per year, number of papers published per year, journal sources, frequently cited journals, high-yield authors, high-yield institutions, national number of papers published, author cooperation network, and world cooperation map. VOSviewer is a bibliometric analysis software used to build and view bibliometric maps. [25] It is based on the principle of collaborative and co-occurrence data and can be used to draw scientific maps in various domains of knowledge. Finally, CiteSpace is an information visualization software developed using Java. [26] It mainly evaluates the literature in a specific field based on co-citation analysis theory and the path-finder algorithm to explore the key paths and knowledge inflection point for the evolution of the considered field and formulate an analysis of the potential dynamic mechanism of the discipline's evolution and the exploration of the frontiers of the discipline's evolution through the drawing of a series of visualization maps.  Figure 1, the Bibliometrix package (version 3.2.1) was used to illustrate the distribution trend of the annual number of published papers. Considering the number of published papers, it can be stated that the academic research conducted in this field from 1991 to 2003 was in its initial stage. Since then, it has grown annually, increasing rapidly from 2016 to 2021, and peaked in 2021 (141 papers), with an annual growth rate of 7.27%. As the articles in 2022 were not included, there was a downward trend. The average number of articles cited per year fluctuates, with the highest average number of citations for each paper reaching 8.8 times per year in 1993 (see Fig. 2). Upon closer inspection, the peak in 1993 was attributed to a paper published by Svensson. [27] This study explored the factors that influenced early death and postoperative complications through a retrospective analysis of patients who underwent thoracoabdominal aortic operations between 1960 and 1991; it received a total of 889 citations.

Features of publication papers.
The documents were published from 288 publication sources, which were ranked by the h-index. The top ten published papers with the highest h-index values are listed in Table 2. We used five basic indicators to define the quantitative characteristics of these sources: h-index, impact factor (IF), total citations of publications, number of publications, and year in which each publication started to publish the articles relevant to the field (PY_start).
As shown in Table 2, if the h-index value is considered, the papers that were published in the Annals of Thoracic Surgery had the largest influence on the research on the postoperative complications of AD published from January 1992 to November 2021, with a total of 37 articles. In addition, it published 97 papers in this research field overall and ranked first among all the journals. The total number of citations of the Journal of Vascular Surgery was 5001, which was the highest. If the IF is considered, the European Journal of Vascular and Endovascular Table 1 Main information about data of all published papers related to research on complications after aortic dissection. Surgery had an IF of 7.069, ranking first among all the journals, followed by Gynecologic Oncology (5.482) and the Journal of Thoracic and Cardiovascular Surgery (5.209). Figure 3 shows the number of published papers for each country. The publications were distributed across 56 countries/regions. The darker the color of the country/region, the higher the number of articles published in that country or region. As shown in Figure 3, the USA, China, and Japan had the most publications. Figure 4 shows the cooperative relationships among countries/regions. From Figure 4 and Table 3, we can see that Germany, Italy, the USA, France, and the United Kingdom cooperate closely. Figure 5 shows the top 20 most relevant authors among all the published papers. The top five authors were J. S. Coselli, Y. Li, H. J. Safi, D. Wang, and D. Pacini (reference numbers 17, 16, 16, 16, and 13, respectively). Figure 6 shows the top 20 authors' published papers over time. Coselli had the highest number of publications (17) during the considered period. His first publication on the subject was in 1991, and his greatest scientific production was registered in 2008. Next, Figure 7 shows the co-authorship network among the authors. The network map had 946 nodes and 1262 connections, and the network density was 0.0028; this indicates that the researchers in this field did not cooperate closely, and only a small number of academic teams were formed.  In the field of research on the postoperative complications of AD, the top three research institutions according to the total number of papers published were Capital Medical University, Fujian Medical University, and Fudan University (Table 4). Further, Figure 8 shows the co-occurrence knowledge map of the research institutions drawn by CiteSpace with a total of 621 nodes. However, there were only 469 connections between the nodes, and the distribution among institutions was relatively scattered, indicating that the research on the postoperative complications of AD lacks close cooperation as a whole.

Frequently cited documents
The number of citations in a publication indicates the popularity of a particular study. Table 5 lists the top 10 globally cited published papers related to research on the postoperative complications of AD, which were ranked based on globally cited articles. Table 6 lists the top 10 locally cited published papers ranked according to the locally cited articles. The top publication was the paper by Svensson (1993), with 889 citations, and its local citations were 29. Most publications with high citation counts were distributed between 1992 and 2008.

Keyword analysis
1.3.4. Co-occurrence analysis. As shown in Figure 9, VOSviewer (version 1.6.18) was used to the conduct co-occurrence analysis of the keywords. A total of 23,454 keywords were extracted from the 1242 papers. We limited the number of occurrences to 25 or more and selected the top 100 keywords with 3901 co-occurrence links distributed in five clusters. Table 7 summarizes the top 20 high-frequency keywords in the research on the postoperative complications of AD. The top five keywords with the highest frequency were "analysis," "incidence," "acute type," "graft," and "risk factor."

Keyword clustering analysis.
Keyword clustering analysis is a process based on keyword co-occurrence analysis  that simplifies the keyword co-occurrence network relationship into a relatively small number of clusters using clustering statistics. This is performed by running CiteSpace, checking the "Purning" option, and selecting the log-likelihood ratio algorithm to determine the research frontier. In the literature, 683 hot spots, 1321 links, and a keyword clustering network map with a density of 0.057 were obtained, as shown in Figure 10. The modularity Q score of the clustering map was 0.7845, and the weighted mean silhouette value was 0.9075. The cluster structure is significant and reasonable. The largest cluster (Cluster #0) out of the 20 clusters was associated with "aortic injury," followed by "postoperative complications" (Cluster #1), "mortality" (Cluster #2), and "acute aortic dissection" (Cluster #3).

Keywords burst detection analysis.
Burst words are keywords that are cited more frequently during a certain period of time and can be used to reflect research trends within that period of time. This is carried out by running CiteSpace and selecting the parameter "Burst Terms." Figure 11 shows that research in related fields has shifted from surgical treatment and utilizing experience to the evidence-based exploration of risk factors and the construction of prediction models to help better manage the related postoperative complications of AD.

Global research trends for the postoperative complications of AD
In this study, the Bibliometrix R-package, VOSviewer, and  [27][28][29][30] proposed measures to effectively control the complications from the management level to prolong survival time, [31] prospectively studied the predictive factors of postoperative respiratory failure, [32] and explored the postoperative outcomes in elderly patients who presented with AD. [33] However, cooperation in this field of research is not high among both researchers and research institutions. Therefore, team cooperation and academic exchanges across disciplines, institutions, and regions should be strengthened in the future.
In 1993, the average number of citations for each paper was the highest, growing 8.8 times per year, which laid the foundation for subsequent research.

Research hotspots
The co-occurrence network of high-frequency keywords presented in Figure 9 shows that the research on the postoperative complications of AD can be divided into three clusters. Based    on the frequency and link strength of keywords in the different areas, we summarized the following research hotspots for the postoperative complications of AD:

Common complications.
Researchers have explored the common postoperative complications of AD, mainly through case reports, retrospective studies, and prospective studies. At present, nervous system-related complications, such as limb ischemia, hemiplegia, and paraplegia, are the most frequently reported complications. Spinal cord ischemia and subsequent paraplegia or direct paraplegia are rare, and catastrophic complications occur during the postoperative treatment of patients who present with acute AD, which seriously affects the postoperative rehabilitation of such patients and significantly increases operative mortality. [34] In 1997, Sakurada reported the case of a 52-year-old female patient with DeBakey type I acute AD who developed paraplegia after emergency surgery. [35] Thereafter, many researchers have reported paraplegia in patients with AD. [36][37][38][39] In addition, some studies have reported cases of internal leakage, [40,41] secondary dissection, [42] and hypoxemia [43] post AD. Svensson was the first to perform a retrospective analysis of 1509 patients who underwent thoracoabdominal aortic repair between 1960 and 1991. [27] The results showed that the overall incidence of paraplegia or paraparesis was 16% (234/1509), and kidney failure (posterior creativity level > 3 mg/dL or diagnosis) occurred in 18% (269/1509) of the patients. Coselli [28] also retrospectively analyzed 372 patients with thoracoabdominal aneurysms, 25% (93/372) of whom had AD, to understand the incidence of renal failure and postoperative neurological impairment. An increasing number of researchers have understood the postoperative complications of AD through retrospective studies. [44][45][46][47][48][49] Relatively few prospective studies have focused on the risk factors associated with the postoperative complications of AD [32,50] and the implementation effect of the new protocol. [51,52] Chen et al [53] conducted a prospective study on 62 patients with type B aortic dissection from 1999 to 2005 to explore the postoperative clinical results of patients at different stages of the disease. Williams used a detailed prospective clinical database to explore the incidence and outcome of the

Risk factors.
Exploring the effect of the risk factors for the early recognition of complications and the improvement of patient prognosis is crucial. Many researchers have explored the factors that influence the postoperative complications of AD through retrospective cohort studies. Khoynezhad et al [55] retrospectively analyzed 153 patients who underwent 184 TEVARs between 1998 and 2005 to explore the risk factors associated with the neurologic deficits that occur after TEVAR. The incidence of stroke and spinal cord injury after TEVAR was 4.3% (8/184). Further, the risk factors associated with stroke are obesity, intraoperative blood loss, and vascular embolism. Aneurysms as an underlying pathology, the use of an iliac conduit, and the coverage of the hypogastric artery were all associated with spinal cord injury. Liu et al [56] explored the risk for delirium after type A AD in 100 patients who underwent Sun's procedure between 2014 and 2016 through a retrospective study. The results showed that cerebrovascular history, duration of surgery and cardiopulmonary bypass, postoperative hypoxia, and intubation time were independently associated with the development of delirium. Svensson conducted a prospective study with 98 patients who underwent thoracoabdominal aortic aneurysm repair (35% of patients presented with AD) to explore the independent predictors of respiratory failure. The   results showed that chronic pulmonary disease and cardiac and renal complications are independent predictors of respiratory failure. In patients with chronic pulmonary disease, the only independent predictor was FEF 25 . [32] Conzelmann et al [50] conducted a multi-center prospective study on 2137 patients with AAAD using the German Registry for Acute Aortic Dissection Type A from 2006 to 2010 to explore the risk factors for neurological dysfunction, and the study found that new postoperative neurological dysfunction was associated with extensive malperfusion syndrome, supraaortic vessel dissection, and operative time. In addition, some scholars have conducted research on the unique traumatic stress caused by surgery, such as post-traumatic stress disorder (PTSD). Lin conducted a prospective cohort study with 224 patients who presented with AAAD between 2017 and 2019 to explore the incidence and risk factors of PTSD. The incidence of PTSD was 21.4%. Depressive symptoms and women are risk factors related to PTSD, while optimism is a protective factor in patients who present with  AAAD. [57] In recent years, an increasing number of researchers have devoted themselves to exploring the risk factors for the postoperative complications of AD, such as delirium, [56,58] acute respiratory distress syndrome, [59] hypoxemia, [60] distal segment aortic enlargement, [61] distal stent graft-induced new entry, [62] acute kidney injury, [63,64] and hepatic dysfunction. [65] 3.4.2. Management of complications. There are many kinds of postoperative complications of AD that affect patients' rehabilitation, prognosis, and quality of life after discharge and even endanger their lives. Therefore, the effective identification and management of complications is a problem that has been explored by researchers. De Santo et al [66] were the first to evaluate the effect of continuous pulmonary perfusion during retrograde cerebral perfusion in a prospective series of 22 AAAD patients. The results showed that continuous pulmonary perfusion had a positive effect on postoperative pulmonary function, which was supported by the enhanced preservation of postoperative pulmonary gas exchange (PaO 2 /FiO 2 ratio) and reduction of ventilator support. Roseborough et al [67] reported a case of acute cerebral hypoperfusion in a 66-year-old patient with AAAD postoperatively. They reported for the first time that an ischemic stroke caused by the malperfusion of arch vessels was successfully reversed by stenting of the involved arch vessels. The patients were followed up with, and their neurological examinations were normal and without abnormal perfusion. Fleck et al [68] described a case report study and showed that cerebrospinal fluid drainage was an effective treatment for paraplegia after stent graft implantation for acute type B aortic dissection, which was also proven by subsequent studies. [69,70] A systematic review and meta-analysis published by Zhang et al [71] on the prevention of spinal cord ischemia through prophylactic cerebrospinal fluid drainage after TEVAR demonstrated that prophylactic cerebrospinal fluid drainage can effectively prevent spinal cord ischemia in patients after AD. Li et al [72] proposed a novel simplified thrombo-inflammatory prognostic score to predict in-hospital complications in patients with AAAD, which has great clinical significance for identifying high-risk patients. In addition, Liu et al [73] developed a nomogram combined with metabolic acidosis to predict high-risk acute type B aortic dissection patients with organ hypoperfusion after TEVAR.
Recently, many scholars have committed to developing new technologies to prevent or minimize the harm caused by complications [74] and, through the establishment of models or specific algorithms, to identify and predict the occurrence of complications early so as to better realize individualized risk assessment and perioperative management. [75][76][77][78][79]

Research frontiers
The fact that the city burst time of the keywords in Figure 11, such as "meta-analysis," "risk factor," "impact," "international registry," "survival," and "predictor," has continued till 2022 and is still ongoing demonstrates that these directions have great potential. In recent years, the research focus has shifted from surgical treatment and experience utilization to the evidence-based exploration of risk factors and the construction of prediction models. Buth et al [80] prospectively enrolled 606 patients (215 with AD) using the European Collaborators on Stent/Graft Techniques for Aortic Aneurysm Repair database to assess the incidence and risk factors for paraplegia or paraparesis and intracranial stroke during the perioperative period. Eggebrecht et al [81] explored the incidence and patient outcomes of retrograde ascending AD after TEVAR using data from 28 centers participating in the European Registry on Endovascular Aortic Repair Complications from 1995 to 2008. Jakob et al explored the incidence of postoperative complications and clinical outcomes for patients according to the International E-Vita Open Registry. [82,83] However, in these studies, the patients with AD were only explored as a sub-category of the research objective. In 2012, Conzelmann et al [50] carried out a multi-center prospective study based on the German Registry for Acute Aortic Dissection Type A to explore the risk factors for neurological dysfunction during the perioperative period of AAAD. Scholars have subsequently used the International Registry of Acute Aortic Dissection [84,85] and the Nordic Consortium for Acute Type A Aortic Dissection [86,87] to understand the incidence of the postoperative complications of AD and its impact on prognosis.
In addition, an increasing number of researchers have carried out meta-analysis to understand the current postoperative complications of AD [88][89][90] and, through retrospective or prospective research, to comprehend the risk factors of complications [61,63,64] to predict the occurrence of complications, which has laid the foundation for the enhanced prognosis of patients with AD, the improved quality of life, and the provision of better clinical management.

Conclusions
In this study, we used the Bibliometrix R-package, VOSviewer, and CiteSpace software to visually analyze 1242 studies on the postoperative complications of AD published between 1991 and 2022, which includes the publication trend, publication source, core author group, country of publication, high-yield institutions, frequently cited articles, and keyword analysis of related studies, to identify the current research hotspots and development trends in this field. This is also the first bibliometric analysis of global publications on the postoperative complications of AD. The results show that the number of articles in this field is increasing annually, indicating that an increasing number of researchers are paying attention to this field. The current research hotspots focus on three areas: common postoperative complications of AD, exploration of the related risk factors, and management of complications. Future research could focus on identifying risk factors through meta-analysis and using a multicenter database for AD as well as on building relevant models to predict the development of complications to facilitate the clinical management of AD patients. In addition, there is still a lack of cooperation between countries. In the future, we should continue to carry out interdisciplinary, inter-institutional, and cross-regional team cooperation and academic exchanges and publish more influential research to better help promote patients' postoperative rehabilitation and improve their quality of life after discharge.

Limitations
The potential limitation of our study lies in the fact that the publications related to the postoperative complications of AD were only extracted from the WoSCC database and Scopus, PubMed, Medline, Embase, and other databases were not considered; therefore, the data in the present study might not be comprehensive. In addition, we only selected the articles that were published in English, which resulted in a language bias.